1. Field of the Invention
The present invention relates to a malfunction diagnosis device for an exhaust gas purification system of an internal combustion engine and a malfunction diagnosis method therefor.
2. Description of the Related Art
In the related art, there has been proposed an exhaust gas purification system in which a selective catalytic reduction catalyst is disposed in the exhaust passage of an internal combustion engine and which includes a urea solution adding device for adding urea solution to exhaust gas that flows upstream of the selective catalytic reduction catalyst.
In the above-mentioned system, ammonia slip (a phenomenon that ammonia (NH3) passes through a selective catalytic reduction catalyst) becomes more pronounced as degradation of the selective catalytic reduction catalyst proceeds. Accordingly, it is necessary to correct the addition quantity of urea solution in accordance with the degree of degradation of the selective catalytic reduction catalyst.
As a method of diagnosing the degradation of a selective catalytic reduction catalyst, there has been proposed a method which integrates the operating time during which the temperature of exhaust gas that flows into the selective catalytic reduction catalyst is equal to or higher than a specified temperature, and diagnoses the degree of degradation of the selective catalytic reduction catalyst to be higher as the integrated time becomes longer (see, for example, Japanese Patent Application Publication No. 2006-242094 (JP-A-2006-242094)).
Incidentally, although the diagnosis method according to the related art mentioned above can be carried out without adding a NOx sensor, an ammonia concentration sensor, or the like, there is a possibility that accurate diagnosis cannot be performed depending on the usage conditions, the operating conditions of the internal combustion engine, and the like.
While there has been also proposed a method with which diagnosis is done by detecting the quantity (concentration) of nitrogen oxides (NOx) or ammonia (NH3) that flows out of a selective catalytic reduction catalyst at the time when urea is supplied to the selective catalytic reduction catalyst, it is necessary to specify whether the difference between the detected value and a normal value is due to malfunction or degradation of the urea adding device, malfunction or degradation of the sensor, or degradation of the selective catalytic reduction catalyst.